API / Security:
- Add api/_helpers.php: shared send_security_headers(), verify_origin(),
get_hmac_secret(), check_rate_limit(), read_json_locked(), write_json_locked()
- shorten.php: remove Access-Control-Allow-Origin:*, restrict to same-origin,
rate-limit 20 req/h per IP, atomic JSON read+lock, HMAC secret from file
- verify.php: rate-limit GET (30/min) and POST (10/h) per IP, atomic lock,
prevent overwriting existing proofs, origin check on POST
- node.php: fix rate limit from 1000 to 60 req/min, add security headers,
origin check
- check-short.php: add security headers, re-derive signature server-side
- s.php: use file-based HMAC secret via get_hmac_secret(), hash_equals()
for timing-safe comparison
Service Worker:
- sw.js: navigation requests (mode=navigate) never served from cache;
network-first with offline fallback to prevent stale invoice state
Documentation (honest claims):
- README: tagline "No backend" -> "No tracking"; new Architecture table
listing exactly what server sees for each feature; Security Model section
- index.html: meta description and footer updated from "No Backend" to
"Minimal Backend"
- i18n.js footer: already updated in previous commit
